Semiconductor image sensors are used to sense radiation such as lights. Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) are widely used in various applications such as digital camera or mobile phone camera. These devices utilize an array of pixels located in a substrate, including photodiodes and transistors that can absorb radiation projected toward the substrate and convert the sensed radiation into electrical signals.
A back side illuminated (BSI) image sensor device is one type of image sensor device. These BSI image sensor devices detect lights projected from the backside. A BSI image sensor device has a relatively thin silicon substrate (e.g., a few microns thick) in which light-sensing pixels are formed. The quantum efficiency and the well capacity of the BSI image sensors depend on the size of the radiation-sensing region. The BSI image sensor device has an image sensing grid and a reference pixel normally located around the image sensing grid.
Interference to the reference pixel from ambient environment needs to be reduced in order to achieve a better performance. Noise to the reference pixel becomes an issue to improve the accuracy of the BSI image sensor device.